The present patent application relates to a light inexpensive cross used in mixers for concrete, mortar and similar materials.
The invention has been devised to optimise a very popular technology used for the realisation of automatic mixers for concrete, mortar and other building materials.
Traditional mixers are composed of a large circular tank that contains the mix ingredients, in which multiple sets of three basically vertical rotary mixing arms operate, whose lower part is provided with blades used to continuously mix the mass contained in the tank until the different ingredients are completely blended.
In order to ensure the rotation of the aforementioned arms, a large gearmotor is placed vertically over the tank lid, on whose shaft a crown wheel is splinted to drag into rotation three rotary plates, with each plate associated to one set of three arms.
Each rotary plate, of basically triangular shape, has a quite strong metal structure obtained with casting and characterised in that each corner is provided with a sort of clamp used to block the upper end of the tubular mixing arm.
More precisely, on each corner the plate incorporates a semicircular sleeve (a jaw) in which the upper end of the mixing arm is engaged. The arm is then blocked by means of a second semicircular jaw capable of being tightened with the first fixed jaw with bolts with horizontal axis.
It appears evident that the fixing of the mixing arm to the traditional plate is quite complex. Moreover, this operation is complicated by the fact that it requires the presence of a special anti-rotation cover capable of preventing the mixing arm from accidentally rotating around its axis in the two fixing jaws because of the resistance encountered by the blade during mixing and caused by the presence of lumps in the mix or by the undesired interference with the tank bottom.
In practical terms, the operation needed to rigidly fix the mixing arm to the blade-holding plate is particularly complicated, in view of the fact that during the mixer operation it is often necessary to vary the position of the arms with respect to the tank bottom, by moving them slightly downwards to restore the very short distance between the lower border of the blades of the arms and the bottom of the wall, which is necessary to obtain good-quality mix.
In order to ensure good mixing, the blades that are located at the end of the three arms must graze the bottom of the mixing tank during rotation; this condition, however, is often impossible due to the wear of the blades or tank on the xe2x80x9ccircular pathsxe2x80x9d that are continuously followed by the three rotary blades on the bottom of the tank.
Wear appears quite suddenly when the hardest ingredients of the mix (for example gravel) get between the lower border of the blades and the bottom of the tank, thus creating significant interference.
As soon as wear creates an excessive space between the border of the blades and the bottom of the tank, the three blade-holding arms must be moved downwards slightly to restore the correct position with respect to the tank bottom.
As mentioned above, the adjustment of the blade position is extremely uncomfortable and complicated with the current technology, since it is necessary to remove the anti-rotation cover from the top of the arm to be adjusted and separate the two fixing jaws.
Once these operations have been carried out, the position of the arm can be adjusted downwards. Then, the fixing jaws must be coupled and the anti-rotation cover replaced.
Traditional mixers are also impaired by another significant disadvantage, which is basically related to the frequent damage of the most delicate and expensive parts, that is the crown wheel and the gearmotor that drags it into rotation.
When rotating near the tank bottom, the blade of the mixing arm can often interfere vigorously with particularly compact lumps of material or even with the bottom wall of the tank in the unload sector.
If we consider that the rotary blade is provided with certain inertia, it appears evident that such an impact may cause a violent impact on the blade that also affects the mixer parts joined to the blade, that is to say the rotary holding plate, the crown wheel that drags the plate into rotation and the gearmotor that actuates the crown wheel, respectively.
In the worst case the impact can easily cause the irreversible breakage of the gears of the crown wheel or gearmotor, thus requiring the replacement of these sophisticated parts.
The specific purpose of the invention is to realise a cross used to support the mixing arms and capable of being applied to the crown wheel of traditional mixers in replacement of the heavy die-cast triangular plates.
As illustrated below, the new cross of the invention is much lighter and cheaper that traditional rotary arm-holding plates.
This first objective has been achieved thanks to the fact that the cross does not have the traditional triangular shape and is not obtained with casting. On the contrary, the new cross has a star-like shape, with three spokes projecting at 120xc2x0 from a central hub, and is obtained from a plate with oxygen lance cutting technique.
Oxygen lance cutting is undoubtedly easier and less expensive than casting and provides crosses with the same efficiency and resistance as traditional crosses.
The new cross of the invention provides rapid and easy fixing of mixing arms, which are also obtained with oxygen lance technique, while completely preventing the mixing arms from rotating around their axis even in the presence of strong interference encountered in the mass to be mixed.
This is because each mixing arm is no longer fixed by means of a xe2x80x9cdouble jawxe2x80x9d clamp, but with bolts with vertical axis that allow for firmly fixing, with no possibility of rotation, the upper end of each arm against the lower side of the spoke of the cross of the present invention.
Equally important from the practical viewpoint is the fact that in the new cross of the invention the adjustment in height of each mixing arm can be realised directly on the blade and therefore disassembling, adjusting and reassembling the entire mixing arm.
Moreover, the cross of the invention has the intrinsic capability of maintaining the integrity of the crown wheel and gearmotor of the mixer, including in the presence of strong impacts suffered by the blade-holding arms that are joined to it during rotation inside the mixture.
Each arm has been intentionally designed as the xe2x80x9cweakestxe2x80x9d element of the entire mixer structure; this means that, in the case of a violent impact suffered by the blade, the mixing arm breaks immediately, thus dissipating all the energy of the violent counterblow derived from the impact.
The xe2x80x9csacrificexe2x80x9d of the mixing arm prevents the counterblow from discharging directly on the more sophisticated and expensive mechanical parts (crown wheel and gearmotor). This xe2x80x9csacrificexe2x80x9d does not have any significant negative consequence, since the cost of each arm obtained with oxygen lance cutting technique is quite low, and replacement is very rapid and simple, thanks to the blocking systems with bolts.
Still with the aim to make the cross structure more convenient and functional, an alternate embodiment has been devised, without leaving the scope of the present invention.
This version is characterised by the fact that it consists in a simple triangular bearing plate provided with a central hub to be coupled to the crown wheel shaft and three independent separate spokes, capable of being fixed to the triangular bearing plate by means of suitable bolts, it being provided that each spoke can support a blade-holding arm of the type illustrated above.
It appears evident that this version of the cross aims at avoiding the difficulties and damages caused by the possible breakage of one of the spokes. In the case of breakage of one of the spokes of the xe2x80x9cmonolithicxe2x80x9d cross illustrated above, the entire cross must be disassembled and replaced according to a procedure that is quite complicated for typical final users of concrete mixers.
In the latter version of the cross according to the present invention, the possible breakage of one of the three independent spokes simply involves the disassembly of the broken spoke and its replacement with a new spoke. It appears evident that such an operation can be carried out even by inexpert users, not only because it simply requires the loosening and tightening of the bolts, but also because it is no longer necessary to remove the coupling of the cross hub with the crown wheel shaft and restore the correct coupling of the hub of the new cross with the same shaft.